Primary cultures of RTE cells are exposed to carcinogens on Day 1, then morphologically altered epithelial foci are scored at Day 30. Tracheal epithelial cells are isolated from 8 to 12 weeks old male Fisher 344 rats and plated on collagen coated dishes. Initial cytotoxicity tests with the test chemopreventive agent are used to select appropriate concentration ranges for use in the transformation-inhibition assay. From the initial cytotoxicity test, the highest concentration which permits a CFE of 80% or greater compared to control is determined. This concentration and four half-log lower concentrations are included in the chemopreventive test assays. The attached cells are exposed to: media alone, solvent alone, benzo(a)pyrene alone, benzo(a)pyrene plus retinoic acid (or other positive control), or benzo(a)pyrene plus chemopreventive agent. At Day 2 the benzo(a)pyrene is removed and exposure to the chemopreventive agent, retinoic acid (or other positive control), or solvent continued. At 5-7 days three dishes per group are fixed and stained for cytotoxicity determination. At Day 30 all remaining cultures are fixed, stained, and morphologically altered foci scored. Computerized morphometric analysis is being performed on a subset of the agents at various timepoints. The transformation frequency of the various groups are determined, and the agents prioritized for further development. There are 20 chemopreventive agents to be screened by this assay for chemopreventive activity.